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By default, the output format is the text form of a DNS KEY record; the host name is the one included in the key information (or, if that is not available, the output of hostname --fqdn), with a . appended. If information about how the key was generated is available, that is provided as a DNS-file comment. For example (with the key data trimmed down for clarity):
; RSA 2048 bits xy.example.com Sat Apr 15 13:53:22 2000 xy.example.com. IN KEY 0x4200 4 1 AQOF8tZ2...+buFuFn/
The --txt option causes the output to be in opportunistic-encryption DNS TXT record format, with the specified gateway value. Again, generation information is included if available. For example, --txt 10.11.12.13 might give (with the key data trimmed for clarity):
; RSA 2048 bits xy.example.com Sat Apr 15 13:53:22 2000 IN TXT "X-IPsec-Server(10)=10.11.12.13 AQOF8tZ2...+buFuFn/"
No name is supplied in the TXT record because there are too many possibilities, depending on how it will be used. If the text string is longer than 255 bytes, it is split up into multiple strings (matching the restrictions of the DNS TXT binary format). If any split is needed, the first split will be at the start of the key: this increases the chances that later hand editing will work.
The --left and --right options cause the output to be in ipsec.conf(5) format, as a leftrsasigkey or rightrsasigkey parameter respectively. Again, generation information is included if available. For example, --left might give (with the key data trimmed down for clarity):
# RSA 2048 bits xy.example.com Sat Apr 15 13:53:22 2000 leftrsasigkey=0sAQOF8tZ2...+buFuFn/
Normally, the default key for this host (the one with no host identities specified for it) is the one extracted. The --id option overrides this, causing extraction of the key labeled with the specified identity, if any. The specified identity must exactly match the identity in the file; in particular, the comparison is case-sensitive.
The --file option overrides the default for where the key information should be found, and takes it from the specified secretfile.
The need to specify the gateway address (etc.) for --txt is annoying, but there is no good way to determine it automatically.
There should be a way to specify the priority value for TXT records; currently it is hardwired to 10.
The --id option assumes that the identity appears on the same line as the : RSA { that begins the key proper.